The Flange Gall is what is known as an "undescribed" gall. That means that no one has figured out what kind of wasp creates the gall. Flange galls are considered rare, but I have found many of them at Rockville Hills Regional Park. They are on Blue Oak (Quercus douglasii) leaves.
The upperside of the blue oak leaf.
The underside of the leaf where the galls are.
I had thought that these galls started out pink and then turned more brown. But when I saw this leaf, I realized two things: first, that the points on the base of the gall grow as the gall develops; and second, the galls start out white or pale yellow. Next comes pink (see photo 6) and then finally the brown.
Here you can see the older gall has fully developed points on the base, while the others have either no points or slightly protruding ones.
Here's another young gall (on the right) that doesn't have its "points" yet.
Pink galls.
As the galls age, they begin to turn brown.
It took a while to find a second batch of these galls, but now I've located them in many areas of the park. To give you an idea of how tiny the galls are, the entire length of this leaf is about 1 3/4-inches long.